PURPOSE: To provide a method wherein a cutting load can be observed even in an acceleration/deceleration period at the time of start and finish of cutting, and the judgement of tool abrasion can be stably performed.

CONSTITUTION: This method is applied to observe a cutting load based on the load current value of the servo motor or main spindle motor of a feed shaft in a lathe or the other cutting machine. A value (b) obtained by subtracting a current value required for accelerating/decelerating a motor from the detected value (a) of motor current is correctedly operated. After this correcting, the current value (b) is adapted as the observing object of a cutting load to observe a peak or a mean value. Tool abrasion is judged by observing a mean value, that is, comparing the mean value of the corrected current value (b) of a set observation section TO part with set values M1 and M2. The set observation section TO is set, e.g. every one pass of cutting.